Psoriasis is a chronic skin condition, which affects approximately 2 % of the world
population. A typical clinical occurrence is excessive peeling of the epidermis. For
treatment we use dermatics and phototherapy and in severe cases systemic therapy. One of
the active ingredients for dermal use is a synthetic analog of vitamin D3 calcipotriol, which
inhibits the excessive activity of T lymphocytes and skin cells, therefore reducing skin
peeling. There is little available data on its stability and because of incompatibility, we
cannot combine it with salicylic acid. Due to the keratolytic properties of salicylic acid we
would achieve a synergistic effect of the combination (treatment of psoriasis by two
pathways), as well as increased calcipotriol penetration into deeper layers of the skin.
Since literature data on calcipotriol stability is quite limited, we systematically studied its
stability and evaluated its compatibility with salicylic acid. Evaluation was based on a
stability-indicating high-performance liquid chromatography (HPLC-UV) method, which
was beforehand evaluated, thus confirming its suitability for the intended use. A
preliminary stability study of calcipotriol was performed on solutions. We concluded that
calcipotriol is very unstable in aquatic solutions, furthermore, its concentration additionally
declined at elevated temperatures, in the presence of metal ions, oxidants, in acidic and
alkaline conditions, and with the addition of salicylic acid. The stability of calcipotriol was
improved by using methanol as solvent, where even in combination with salicylic acid, it
remained stable in comparison with aquatic solutions.
Subsequently, the destabilizing effect of salicylic acid on calcipotriol was evaluated in
formulations, found in Slovenia. Before preparing the combinations of commercial
preparations with calcipotriol and salicylic acid, a calcipotriol extraction procedure from
ointment and dermal solution Sorel was developed. The destabilizing effect of salicylic
acid was evaluated at different temperatures and concentrations. We found that the
therapeutic concentration of salicylic acid caused total calcipotriol degradation in both
formulations after only one day of storage at room temperature. The stability was improved
with lowering the concentration of salicylic acid, which consequently lowers its efficiency.
Additionally, we established temperature depended stability in both forms of Sorel
medication, which lowered at higher temperatures.
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